1. Field of the Invention
The present invention relates to a backlight unit, a method for driving the same, and a liquid crystal display device using the same; and more particularly, to a backlight unit, in which a feedback voltage received in a controller to adjust a driving voltage to be supplied to a light-emitting diode string is smaller than a difference between the driving voltage and a string voltage, a method for driving the same, and a liquid crystal display device using the same.
2. Discussion of the Related Art
A backlight unit is used as an illuminating device for a display panel. The backlight unit according to the related art uses a light source of cold cathode fluorescent lamp (CCFL). However, the CCFL using mercury therein may cause the environmental contamination. In addition, the CCFL has problems such as low response speed of 15 ms, and low color-realization. In detail, a color-realization ratio of CCFL is lowered by 75% as compared to the color-realization ratio of NTSC. Due to the aforementioned problems of the CCFL, a light-emitting diode (LED) has attracted great attentions as the light source for the backlight unit.
In comparison to the CCFL, the LED is environmentally-friendly, and enables a rapid response by realizing a response speed of several nano-seconds. Also, the LED can be driven by an impulse, and the LED can obtain the color-realization ratio of 80˜100%. Also, if using the LED as the light source for the backlight unit, luminance and color temperature of the backlight unit can be controlled by adjusting light-radiation intensity of the LED.
In the backlight unit using the LED, there are plural LED strings, wherein each LED string includes the plural LEDs electrically connected in series.
FIG. 1 is a circuit diagram illustrating a backlight unit according to the related art.
Referring to FIG. 1, the backlight unit according to the related art includes a driving-voltage supplier 10, an LED string 20, a controller 30, a switching device (Q), and a resistor (Rs).
The driving-voltage supplier 10 generates a driving voltage (Vd) for driving the LED string 20 through the use of input voltage (Vin) supplied from the external under the control of the controller 30; and supplies the generated driving voltage (Vd) to the LED string 20.
For convenience of explanation, FIG. 1 shows only one LED string 20. However, the virtual backlight unit is provided with the plural LED strings which are driven in the same method. The plural LED strings are electrically connected in parallel to an output terminal of the driving-voltage supplier 10.
The LED string 20 includes plural LEDs electrically connected in series between the output terminal of the driving-voltage supplier 10 and the switching device (Q). Each of the plural LEDs is driven by the driving voltage (Vd) supplied from the output terminal of the driving-voltage supplier 10, to thereby emit the light. In this case, as a driving current (Ist) flows in the LED string 20, a voltage drop corresponding to a string voltage (Vst) occurs.
The controller 30 receives a first feedback voltage (Vfb1) from a first node (N1), wherein the feedback voltage (Vfb1) corresponds to a difference between the driving voltage (Vd) and the string voltage (Vst); and adjusts the driving voltage (Vd) by controlling the driving-voltage supplier 10 based on the first feedback voltage (Vfb1).
For example, if the first feedback voltage (Vfb1) is higher than a reference voltage, the controller 30 lowers a voltage value of the driving voltage (Vd) outputted from the driving-voltage supplier 10. Meanwhile, if the first feedback voltage (Vfb1) is lower than the reference voltage, the controller 30 raises the voltage value of the driving voltage (Vd) outputted from the driving-voltage supplier 10. Accordingly, the controller 30 can supply the constant driving voltage (Vd) to the LED string 20.
The controller 30 receives a second feedback voltage (Vfb2) from a second node (N2), wherein the second feedback voltage (Vfb2) corresponds to a voltage drop occurring when the driving current (Ist) flows in the resistor (Rs); and adjusts an amount of the driving current (Ist) flowing in the LED string 20 by controlling the switching device (Q) based on the second feedback voltage (Vfb2).
In the aforementioned backlight unit according to the related art, the first feedback voltage (Vfb1) received in the controller 30 to adjust the driving voltage (Vd) to be supplied to the LED string 20 is the same as the difference between the driving voltage (Vd) and the string voltage (Vst). That is, the first feedback voltage (Vfb1) can be measured by the following equation 1.Vfb1=Vd−Vst,   [Equation 1]wherein ‘Vfb1’ indicates the first feedback voltage; ‘Vd’ indicates the driving voltage; and ‘Vst’ indicates the string voltage.
A manufacturing cost and unit cost of the controller 30 is increased in proportion to a permissible voltage range, that is, the voltage value of the first feedback voltage.